Mozilla is the most popular web browser due to its extensive extension system allowing users to customize it according to their needs. Internet Explorer is the second most popular due to its user-friendly and easy to understand interface. Safari is Apple's proprietary web browser and currently the fastest, with a new version for Mac OS X Leopard. Add-ons and plug-ins can enhance browsers with additional features, games, tools and appearances, making the programs more customizable for users. Popular plug-ins include Adobe Flash for multimedia and Microsoft Silverlight for animations and rich internet applications. The first web browser was nextStep created in 1990-1991 by Tim Berners-Lee and Robert Cailliau.